The present invention relates to an arc-heating type extra-furnace refining apparatus arranged such that electrodes are immersed in slag on molten steel in a ladle to thereby form an arc between the electrodes and the molten steel to heat the molten steel. More particularly, the invention is concerned with an arc-heating type extrafurnace refining apparatus in which the sealability between the covering of the ladle and the electrodes is improved.
In the arc process (hereinafter, referred to simply as "AP") of making an extra-furnace refinement of molten steel tapped from a converter, as shown in FIG. 1, electrodes 8 are immersed in a slag 6 on molten steel 4 charged into ladle 2 and an arc is formed between the molten steel and the electrodes to heat the molten steel. Simultaneously, lance 10 is immersed into the molten steel to thereby introduce a gas into the molten steel to stir the same. In this case, since lid or covering 12 is set on ladle 2, electrodes 8 and lance 10 are inserted into the ladle via insertion holes 14 provided in covering 12. At the top of ladle 2, dust collecting duct 15 is mounted, which is intended to collect exhaust gas containing the dust which is produced at the time of heating the molten steel or stirring the molten steel by bubbling or conducting powder injection.
Meanwhile, since electrodes 8 are kept at a high temperature and supplied with a high voltage, a small gap is allowed to exist between each electrode 8 and covering 12. For this reason, the flame which has been generated in the vicinity of the electrode portions within the ladle comes outside via the gaps. This flame causes an upward flow of the gas in the ladle as indicated in FIG. 1 by arrows which causes atmospheric air to be sucked into the ladle through, for example, seal 16 between covering 12 and ladle 2. As a result, reoxidation of the molten steel takes place in the ladle and, at the same time, the content of nitrogen [N] in the molten steel increases with the result that what is called "pickup" occurs.
In order to prevent the occurrence of the [N] pickup phenomenon, as shown in FIG. 2, refractory board 18 made of ceramic fiber is disposed between electrodes 8 and the covering 12 to thereby seal the gap therebetween. This sealing means, however, has a drawback in that refractory board 18 fails to function as a sealing means in the final half of AP because it is damaged by the flame which has been generated in the beginning stage of arc-heating. For this reason, where the gap between the electrode and the covering is sealed with the use of the refractory board, a [N] pickup of 0.25 to 0.75 ppm/min. still occurs.
On the other hand, a method of blowing Ar gas onto the portions of electrodes 8 in the vicinity of insertion holes 14 to thereby seal the gap between the electrodes and the covering can also be contemplated as a countermeasure. This gas seal, however, fails to have a sufficient sealing function because the Ar gas is pushed upwards by the upward flow of gas coming out of insertion holes 14. For this reason, the conventional sealing means fails to sufficiently prevent the reoxidation of the molten steel as well as not preventing the pickup of [N] and, in addition, requires the use of a large amount of seal gas (Ar gas). In this old sealing means, it is necessary to use Ar gas of, for example, approximately 300 Nm.sup.3 /hour or more and this becomes a factor of increasing the refining cost.